Description:
------------
If we declare a class constant twice or more PHP allows this without any notifications. This should be fixed, because it may bring to confusion. A constant should be constant, thus not allowing to be overriden neither in the same class, nor in child classes.

Currently non-class constants allow to be overriden too, at least they output a notice saying "Constant already defined". Maybe this should be brought up to warning.

Just my thoughts!

Reproduce code:
---------------
<?php
class A {
	const MY_CONSTANT = "A. value_1";
	const MY_CONSTANT = "A. value_2";
	
	public function __construct() {
		echo self::MY_CONSTANT, "\n";
	}
}

class B extends A {
	const MY_CONSTANT = "B. value_1";
	const MY_CONSTANT = "B. value_2";
	
	public function __construct() {
		echo self::MY_CONSTANT, "\n";
	}
}

new A();
new B();
?>

Expected result:
----------------
should display warning, error or something like that.

Actual result:
--------------
A. value_2
B. value_2